With the recent advancement and progress in the field of wireless power transfer (WPT), there is an ever increasing demand for high power transfer efficiency (PTE) of the WPT systems and improved transfer distance for the end-users. However, some existing WPT systems have limited PTE and transfer distance as they take the inductive coupling approach, where the PTE dramatically decreases as the distance between (Tx) and receiver (Rx) coils increases. Alternatively, magnetic resonance coupling (MRC) is used as a mid-range WPT approach, for which the insertion of metamaterials (MTMs) between Tx and Rx coils is exploited to improve efficiency. MTMs are artificially engineered materials that show uncommon electromagnetic properties, such as evanescent wave amplification and negative refractive characteristics, which could be utilized for the enhancement of PTE. In this article, a comprehensive review on recent progresses in the MTM-based WPT systems is reported, where previously reported MTM-based WPT systems are compared in terms of various parameters such as configurations, operating frequencies, dimensions and PTE. Also, the PTEs of these systems were plotted as a function of the normalized transfer distance. This review is expected to provide an insight for understanding the trends of the MTM-based WPT systems and serve as a reference for researchers who work on WPT systems and their applications.

This letter presents a tunable metamaterial (MTM) slab for high-efficiency wireless power transfer (WPT) in misaligned conditions, which operates at 6.78 MHz, an AirFuel Alliance Standard frequency. The tunable MTM slab is located between transmitter (Tx) and receiver (Rx) coils serving as a tunable lens to help focus the magnetic field toward the place where the Rx coil is located. This tunable slab consists of two types of unit cells which can be independently tuned to have proper negative refractive indices. The experimental results show that the tunable MTM slab effectively focuses magnetic fields, thereby improving the power transfer efficiency (PTE) of WPT even in a misaligned condition.

This paper presents a metamaterial (MTM)-integrated high-gain rectenna for RF sensing and energy harvesting applications that operates at 2.45 GHz, an industry, science, medicine (ISM) band. The novel MTM superstrate approach with a three-layered integration method is firstly introduced for rectenna applications. The integrated rectenna consists of three layers, where the first layer is an MTM superstrate consisting of four-by-four MTM unit cell arrays, the second layer a patch antenna, and the third layer a rectifier circuit. By integrating the MTM superstrate on top of the patch antenna, the gain of the antenna is enhanced, owing to its beam focusing capability of the MTM superstrate. This induces the increase of the captured RF power at the rectifier input, resulting in high-output DC power and high entire end-to-end efficiency. A parametric analysis is performed in order to optimize the near-zero property of the MTM unit cell. In addition, the effects of the number of MTM unit cells on the performance of the integrated rectenna are studied. A prototype MTM-integrated rectenna, which is designed on an RO5880 substrate, is fabricated and characterized. The measured gain of the MTM-integrated rectenna is 11.87 dB. It shows a gain improvement of 6.12 dB compared to a counterpart patch antenna without an MTM superstrate and a maximum RF–DC conversion efficiency of 78.9% at an input RF power of 9 dBm. This results in the improvement of the RF–DC efficiency from 39.2% to 78.9% and the increase of the output DC power from 0.7 mW to 6.27 mW (a factor of 8.96 improvements). The demonstrated MTM-integrated rectenna has shown outstanding performance compared to other previously reported work. We emphasize that the demonstrated MTM-integrated rectenna has a low design complexity compared with other work, as the MTM superstrate layer is integrated on top of the simple patch antenna and rectifier circuit. In addition, the number of MTM units can be determined depending on applications. It is highly envisioned that the demonstrated MTM-integrated rectenna will provide new possibilities for practical energy harvesting applications with improved antenna gain and efficiency in various IoT environments.

In this work, a highly efficient metasurface-based multi-scale wireless power transfer (MSWPT) system is presented. The MSWPT is designed to work as a near-field WPT at 6.78 MHz (AirFuel Alliance standard frequency) and far-field WPT at 433 MHz (ISM band for household devices) according to its power transfer distance (PTD). This property that can function in both near-field scale and far-field scale is defined as multi-scale property in this paper. The metasurface slabs which have an electromagnetic (EM) wave focusing characteristic for both frequency bands are integrated with the transmitter (Tx) and receiver (Rx) coils to enhance the power transfer efficiency (PTE) of the MSWPT. The metasurface-based MSWPT system is characterized and measured for both near-field and far-field scales. The measured results exhibit that the PTE of the MSWPT is improved from 5.4 % to 50.1 % with the metasurface slabs at a PTD of 50 cm (near-field scale at 6.78 MHz), i.e. 9.3 times improvement while the PTE at a PTD of 140 cm (far-field scale at 433 MHz) is improved from 2.3 % to 9.4 % with the metasurface slabs, i.e. 4.1 times improvement. In addition, the PTE measurements with the lateral and angular misalignment conditions in the MSWPT system are investigated for practical applications. Results show that the metasurface slabs enhance the PTE of the MSWPT even in the misaligned condition.

This work presents a high-efficiency reconfigurable wireless-power-transfer (WPT) system using fully rollable Tx/Rx coils and a metasurface (MS) screen working at 6.78 MHz, for the first time. The MS screens are placed between the Tx and Rx to magnify the power-transfer efficiency (PTE) of the WPT system. The proposed MS-based WPT can be rolled down or rolled up as required, which allows end-users to use the space more flexibly. In the measurement results, the PTE of the WPT is improved from 13.32% to 32.49% at a power-transfer distance (PTD) of 40 cm with one MS screen, 5.42% to 42.25% at a PTD of 50 cm with two MS screens, 1.78% to 49% at a PTD of 60 cm with three MS screens, 0.85% to 46.24% at a PTD of 70 cm with four MS screens. The measured PTE results indicate that the demonstrated MS screens are greatly effective for magnifying the PTE and the PTD of the WPT. In addition, the measured PTE results in the misaligned condition verify that the MS screens also help increase the PTE of the WPT even in the misalignment condition.

Human beings are naturally mobile and crave freedom. But, for many years, we were leashed to our office desks/homes to make a long-distance phone call. Finally, in 1973, the first-ever cellular phone call was made by Motorola engineer Dr. Martin Cooper using a handmade prototype device that weighed almost 2.5 lb. As Greek philosopher Heraclitus said, “Change is the only constant,” and over the past 50 years, cellular phone technology has undergone a drastic makeover where we drifted from a 2.5-lb cellular phone to a 0.4-lb smartphone (an iPhone 13), from typewriters to digital systems like desktops and laptops, and from paper maps to GPS for navigation. Similarly, as shown in Figure 1, our journey in wireless mobile communications started with 1G with analog data transmission and a data rate at ∼2.4 Kb/s to a full-fledged and operating 4G at a maximum data rate of 1 Gb/s.